[extglob] Make note of use cases for negation.

Addresses issue #192.

scripts/complete.sh

@@ -18,6 +18,17 @@ grep-extglob() {
   grep -E --color '[@?!+*]\(' "$@"
 }
 
+# A very common case is something like:
+#
+# --host|-!(-*)h
+#
+# which matches --host, -h, -ah, but NOT --h.
+#
+# https://github.com/oilshell/oil/issues/192
+grep-extglob-negation() {
+  grep -E --color '!\(' $BASH_COMP ../bash-completion/completions/*
+}
+
 audit() {
   local file=${1:-$GIT_COMP}